Nowadays, as integrated circuits are increasingly developed toward miniaturization, the conventional two-dimensional transistor structures usually fail to meet the practical requirements. Especially, the performance of the conventional two-dimensional transistor structure in high-speed circuitry is unsatisfied because the current driving capability is insufficient. For solving these drawbacks, a fin field-effect transistor (FinFET) structure has been disclosed. FIG. 1 is a schematic view illustrating a FinFET structure according to the prior art. Like the typical FET structure, the FinFET structure of FIG. 1 comprises a substrate 10, a source 11, a drain 12, a gate insulator layer 13 and a gate conductor layer 14. However, since a channel (not shown) between the source 11 and the drain 12 is covered by the gate insulator layer 13 and the gate conductor layer 14, three surfaces are utilized to provide more current paths. In other words, the FinFET structure has better current driving capability than the typical FET structure. However, it is found that downscaling and optimizing the FinFET structure is a challenge.
Therefore, there is a need of providing an improved fin field-effect transistor structure to obviate the drawbacks encountered from the prior art.